The present invention relates to emitter coupled logic and current mode logic circuits and, more particularly, to diode load emitter coupled logic circuits (DLECL). DLECL circuits are known in the art which use diodes as the load elements of the emitter coupled logic circuit to provide a minimum logic output voltage swing to thereby increase the operation speed of the logic circuit. Most, if not all, DLECL circuits are operated differentially, i.e., the input logic signals are differentially applied to the emitter coupled input stage of the logic circuit. For example, feedback counters are generally constructed with flip-flops connected in series with one or more outputs that are fed back to the first flip-flop of the counter through a DLECL gate. These outputs to the gate are differentially coupled thereto. Another example of a differentially coupled DLECL circuit is a modulus divider described in U.S. Pat. No. 4,585,957.
A common problem with operating DLECL circuits differentially is that this requires series gating, i.e., the stacking of ECL gates to obtain the necessary logic function. Although series gating may conserve current requirement it requires at least an additional diode voltage drop between the power supply which thus limits the minimum supply voltage and also requires the driving output to be level shifted which is usually accomplished using emitter followers which use additional current. A solution or alternative method is to use a single-ended logic input to the DLECL instead of differential applied logic input. This however requires a reference or threshold voltage that is applied to one input of the gate in order to have the gate logically switch output operating states in response to the logic input varying above and below the reference voltage. This reference voltage must ideally be centered between the logic output signal swing.
Hence, a need exists for a threshold voltage generator that can provide the reference voltage required to operate DLECL circuits in a single-ended operation mode.